Image forming apparatus having light emission control and image forming method

ABSTRACT

An image forming apparatus includes a plurality of print heads, a sensor, a processor, and an image forming unit. The plurality of print heads are arranged in parallel. The sensor detects a color shift amount of each color to be printed by the plurality of print heads in a main scanning direction based on an output result of a test pattern by the plurality of print heads. The processor sets a color shift correction value for correcting the color shift amount on the basis of a color having a largest color shift amount. The processor controls light emission of the plurality of print heads based on the color shift correction value and image data. The image forming unit forms an image based on the image data on a sheet by light emission of the plurality of print heads.

FIELD

Embodiments described herein relate generally to an image formingapparatus and an image forming method.

BACKGROUND

Electrophotographic printers (hereinafter, printers) equipped with printheads are widely used. For example, the print head includes a pluralityof light emitting elements such as a light emitting diode (LED) or anorganic light emitting diode (OLED). A photoconductive drum is exposedto the light output from the plurality of light emitting elements, alatent image is formed on the photoconductive drum, and an imagecorresponding to the latent image is printed on a sheet such asrecording paper.

For example, a print head is provided with light emitting elementscorresponding to 15,400 pixels, the arrangement of the light emittingelements correspond to a main scanning direction, and the movement ofthe print head back and forth corresponds to a sub-scanning direction.The print head is mounted on a housing of a printer with high-precisionpositioning, but may be slightly displaced. The influence of this slightdisplacement results in the displacement of an image on a sheet in themain scanning direction.

A color printer includes a plurality of print heads arranged in parallelin the main scanning direction, and the plurality of print headscorrespond to a plurality of colors. In the color printer, the influenceof slight mounting displacement of each print head with respect to thehousing of the printer and the influence of relatively slightdisplacement of each print head result in the displacement of an imageon a sheet in the main scanning direction and color shift of each color.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating an example of a schematicconfiguration of an image forming apparatus according to an embodiment;

FIG. 2 is a diagram illustrating displacement correction of the imageforming apparatus according to the embodiment;

FIG. 3 is a flowchart illustrating an example of the displacementcorrection according to the embodiment;

FIG. 4 is a diagram illustrating color shift correction; and

FIG. 5 is a flowchart illustrating an example of the color shiftcorrection according to the embodiment.

DETAILED DESCRIPTION

In the embodiments presented herein, it is possible to reduce the effectof the relative displacement of each print head by making the width ofeach print head employed in a color printer sufficiently long withrespect to the sheet width, and by controlling the light emission of aplurality of optical elements included in each print head.

In general, according to one embodiment, an image forming apparatusincludes a plurality of print heads, a sensor, a processor, and an imageforming unit. The plurality of print heads are arranged in parallel. Thesensor detects a color shift amount of each color to be printed by theplurality of print heads in a main scanning direction based on an outputresult of a test pattern by the plurality of print heads. The processorsets a color shift correction value for correcting the color shiftamount on the basis of a color having a largest color shift amount sothat an image of each color in the main scanning direction falls withina range of a valid image area commonly covered by the plurality of printheads. The processor controls light emission of the plurality of printheads based on the color shift correction value and image data. Theimage forming unit forms an image based on the image data on a sheet bythe light emission of the plurality of print heads.

Hereinafter, an example of the image forming apparatus according to theembodiment will be described with reference to drawings. In eachdrawing, the same reference numeral is given to the same configuration.

FIG. 1 is a block diagram illustrating an example of a schematicconfiguration of the image forming apparatus according to theembodiment.

For example, an image forming apparatus 1 is a multi-function peripheral(MFP). As illustrated in FIG. 1, the image forming apparatus 1 includesa control unit 101 (controller), a read only memory (ROM) 102, a randomaccess memory (RAM) 103, a nonvolatile memory 104, a communicationinterface (I/F) 105, a control panel 106, an image data bus 110, animage reading unit 111 (reader), an image processing unit 112 (imageprocessor), a page memory 113, an image data transfer control unit 114(image data transfer controller), an image forming unit 115, a printhead 116 (including a plurality of print heads, each corresponding to acolor), a displacement sensor 117, a mechanical control driver 118, atemperature sensor 119, and the like.

The ROM 102, the RAM 103, the nonvolatile memory 104, the communicationI/F 105, the control panel 106, the image data transfer control unit114, the displacement sensor 117, the mechanical control driver 118, thetemperature sensor 119, and the like are connected to the control unit101. The control unit 101 includes one or more processors, and theprocessor is a central processing unit (CPU), a micro processing unit(MPU), a digital signal processor (DSP), or a combination thereof.

The control unit 101 controls operations such as image reading, imageprocessing, and image formation according to various programs stored inat least one of the ROM 102 or the nonvolatile memory 104. The controlunit 101 also controls operations such as image data transfer and lightemission of the plurality of print heads according to various programsstored in at least one of the ROM 102 or the nonvolatile memory 104. Forexample, the processor controls light emission of the plurality of printheads based on image data, a color shift correction value, and adisplacement correction value according to various programs.

The ROM 102 is a non-transitory computer-readable storage mediumfunctioning as a program memory, which stores various programs.

The RAM 103 functions as a work memory, and temporarily stores a programread from the ROM 102 and temporarily stores data handled by theprogram.

The nonvolatile memory 104 is a non-transitory computer-readable storagemedium, which stores various programs, temporarily stores programs readfrom the ROM 102, and temporarily stores data handled by the programs.

Further, the nonvolatile memory 104 stores a displacement correctionvalue for correcting a displacement amount of each print head withrespect to a regular mounting position. The displacement correctionvalue is a correction value for correcting a displacement amount of animage in the main scanning direction with respect to a sheet. Thenonvolatile memory 104 stores a color shift correction value accordingto the relative displacement of the mounting position of each printhead. The color shift correction value is a correction value forcorrecting a color shift amount of each color to be printed by theplurality of print heads in the main scanning direction.

The communication I/F 105 outputs various information to the outside andinputs various information from the outside. The communication I/F 105acquires image data.

The control panel 106 includes an input unit such as a touch panel and adisplay unit such as a display and receives operation inputs from a userand a service person. The control panel 106 is an interface foracquiring a displacement correction value for correcting a displacementamount of an image with respect to a sheet in the main scanningdirection.

The control unit 101, the image reading unit 111, the image processingunit 112, and the page memory 113 are connected to the image data bus110, and the image data bus 110 transfers image data therebetween.

The image reading unit 111 optically reads a document image, acquiresimage data corresponding to the document image, and outputs the acquiredimage data to the image processing unit 112.

The image processing unit 112 performs various image processing such ascorrection on the image data acquired via the communication I/F 105 orthe image data acquired by the image reading unit 111.

The page memory 113 includes a page memory 113-Y (Yellow), 180-M(Magenta), 180-C (Cyan), and 180-K (Key plate). The page memories 113-Y,180-M, 180-C, and 180-K store the image data of each color processed bythe image processing unit 112. The image data transfer control unit 114is connected to the page memories 113-Y, 180-M, 180-C, and 180-K.

The image data transfer control unit 114 transfers image data of eachcolor output from each of the page memories 113-Y, 180-M, 180-C, and180-K to the image forming unit 115.

The image forming unit 115 includes a print head 116 or the like, andthe print head 116 includes print heads 116-Y, 116-M, 116-C, and 116-K.Image data (Y) output from the page memory 113-Y is transferred to theprint head 116-Y, image data (M) output from the page memory 113-M istransferred to the print head 116-M, image data (C) output from the pagememory 113-C is transferred to the print head 116-C, and image data (K)output from the page memory 113-K is transferred to the print head116-K.

For example, the control unit 101 edits image data on the page memories113-Y, 180-M, 180-C, and 180-K according to a print position. Thecontrol unit 101 may edit the image data on the page memories 113-Y,180-M, 180-C, and 180-K according to the print head.

The image forming unit 115 forms an image based on the image datatransferred from the page memories 113-Y, 180-M, 180-C, and 180-K to theprint heads 116-Y, 116-M, 116-C, and 116-K. That is, the image formingunit 115 forms an image according to the light emission (state of lightemission and extinction) of light emitting elements of the print head116 based on the image data. In the present embodiment, the imageforming unit 115 forms an image corresponding to the light emission ofthe light emitting elements of the print head 116 based on at least oneof the displacement correction value and the color shift correctionvalue, in addition to the image data.

The control unit 101 inputs image data of a test pattern into the pagememories 113-Y, 180-M, 180-C, and 180-K, and the image forming unit 115outputs a test pattern. The displacement sensor 117 detects the testpattern output on the transfer belt 103 and outputs a detected signal tothe control unit 101.

The control unit 101 detects the displacement amounts of the print heads116-Y, 116-M, 116-C, and 116-K with respect to regular mountingpositions from the input of the displacement sensor 117. The controlunit 101 sets displacement correction values for correcting thedisplacement amounts of the print heads 116-Y, 116-M, 116-C, and 116-Kso that the image of each color in the main scanning direction fallswithin a range of a valid image area commonly covered by the print heads116-Y, 116-M, 116-C, and 116-K.

For example, based on the displacement amount of the print head 116-Kcorresponding to a reference color (K) among the print heads 116-Y,116-M, 116-C, and 116-K, the control unit 101 sets a displacementcorrection value for each color so that the image in the main scanningdirection falls within the range of the valid image area. A serviceperson or the like visually checks the print result of the test patternformed on the sheet by the image forming unit 115 and inputs adisplacement correction value via the control panel 106. The controlpanel 106 acquires the input displacement correction value, and thecontrol unit 101 sets the acquired displacement correction value.Alternatively, the control unit 101 changes the displacement correctionvalue acquired by the control panel 106 so that the image of each colorin the main scanning direction falls within the range of the valid imagearea. When the image in the main scanning direction is out of the rangeof the valid image area, the control unit 101 changes the displacementcorrection value acquired by the control panel 106 to a predeterminedupper limit. The nonvolatile memory 104 and the like store thedisplacement correction value.

The control unit 101 recognizes a color shift amount, which is arelative displacement amount of each color detected by the displacementsensor 117, and sets a color shift correction value corresponding to thedisplacement amount. For example, the control unit 101 sets a colorshift correction value for correcting the color shift amount on thebasis of a color having a largest color shift amount so that the imageof each color in the main scanning direction falls within the range ofthe valid image area commonly covered by the print heads 116-Y, 116-M,116-C, and 116-K. The control unit 101 controls the light emission ofthe print heads 116-Y, 116-M, 116-C, and 116-K based on the image dataand the color shift correction value. The nonvolatile memory 104 storesthe color shift correction value.

The mechanical control driver 118 controls operations of a motor and thelike necessary at the time of printing according to instructions fromthe control unit 101. The control unit 101 outputs a selection signalfor selecting one sheet cassette from a plurality of sheet cassettesthrough the mechanical control driver 118 based on the information onthe sheet cassette specified from the control panel 106 and theinformation on the loaded sheet cassette provided from the mechanicalcontrol driver 118. The mechanical control driver 118 selects one sheetcassette from a plurality of sheet cassettes according to the selectionsignal from the control unit 101.

The temperature sensor 119 detects internal temperature and notifies thecontrol unit 101 of the temperature detection result. The temperaturesensor 119 detects an external temperature (environmental temperature)and notifies the control unit 101 of the temperature detection result.For example, the control unit 101 controls the execution of the colorshift correction based on the temperature detection result from thetemperature sensor 119.

Displacement Correction of Image with Respect to Sheet

Next, the displacement correction of an image with respect to a sheet inthe main scanning direction will be described.

FIG. 2 is a diagram illustrating displacement correction of the imageforming apparatus according to the embodiment, and is a diagramillustrating an output example of a test pattern. In FIG. 2, for ease ofexplanation, among the plurality of print heads, the print head 116-Kcorresponding to the reference color (K) is illustrated, andillustration of the print heads 116-Y, 116-M, and 116-C corresponding toother colors is omitted.

For example, the print heads 116-Y, 116-M, 116-C, and 116-K of the imageforming apparatus include light emitting elements corresponding to15,400 pixels. The print heads 116-Y, 116-M, 116-C, and 116-K of theimage forming apparatus are arranged in parallel and attached to thehousing of a printer by high-precision positioning. However, inactuality, the positions thereof may be slightly displaced. For example,a displacement of several to several tens of pixels may occur.

The control unit 101 instructs displacement correction at apredetermined timing and controls light emission of the print head 116based on the image data of the test pattern. For example, the controlunit 101 periodically instructs displacement correction. Alternatively,the control unit 101 instructs displacement correction according to theelapsed time from previous displacement correction.

The image forming unit 115 outputs a test pattern on the transfer belt103 based on an instruction for correcting the displacement and emissioncontrol of the print head 116 according to the image data of the testpattern. Further, the image forming unit 115 outputs a test pattern onthe sheet based on the test pattern output on the transfer belt 103. Thedisplacement sensor 117 detects the test pattern output on the transferbelt 103 and outputs a detected signal to the control unit 101. Thecontrol unit 101 detects a valid image area commonly covered by theplurality of print heads based on the detected signal.

For example, a service person or the like determines a displacementcorrection value for correcting the displacement of an image withrespect to a sheet in the main scanning direction while looking at atest pattern on the sheet illustrated in FIG. 2 and inputs thedisplacement correction value via the control panel 106. The controlpanel 106 acquires the input displacement correction value and notifiesthe control unit 101 of the acquired value.

The control unit 101 calculates, for example, a main scanning printposition of the reference color (K) based on the displacement correctionvalue acquired via the control panel 106 and determines whether or notthe calculated main scanning print position falls within the range ofthe valid image area. If the control unit 101 determines that thecalculated main scanning print position is out of the valid image area,the control unit 101 changes the correction value to the upper limitvalue so that the calculated main scanning print position falls withinthe range of the valid image area.

The control unit 101 sets a pixel corresponding to a leftmost lightemitting element among the plurality of light emitting elements of theprint head 106 as an origin and determines how many light emittingelements of the plurality of light emitting elements correspond to theimage in the main scanning direction. That is, the control unit 101determines which light emitting element of the plurality of lightemitting elements corresponds to a left end of the image. The followingis an example of main scanning print position calculation when a K coloris used as a reference color.

Main scanning print position (K color)=((image size of print head116-K)−(maximum sheet size))+2+((reference value 128 (dot))−displacementcorrection value of main scanning position between sheet and image)

Main scanning print position (Y color)=Main scanning print position (Kcolor)+((reference value 128 (dot))−main scanning color shift correctionamount of Y color with respect to K color)

Main scanning print position (M color)=Main scanning print position (Kcolor)+((reference value 128 (dot))−main scanning color shift correctionamount of M color with respect to K color)

Main scanning print position (C color)=Main scanning print position (Kcolor)+((reference value 128 (dot))−main scanning color shift correctionamount of C color with respect to K color)

FIG. 3 is a flowchart illustrating an example of displacement correctionof the image forming apparatus according to the embodiment.

As illustrated in FIG. 3, the control unit 101 instructs displacementcorrection to occur at a predetermined time and controls light emissionof the print head 116 based on the image data of the test pattern. Theimage forming unit 115 outputs a test pattern based on the instructionfor correcting the displacement (ACT 11). That is, the image formingunit 115 outputs a test pattern on the transfer belt 103 based on thelight emission control of the print head 116 according to the image dataof the test pattern and further forms a test pattern on a sheet based onthe test pattern output on the transfer belt 103.

For example, a service person or the like determines a displacementcorrection value for correcting the displacement of an image withrespect to a sheet in the main scanning direction while looking at atest pattern on the sheet and inputs the displacement correction valuevia the control panel 106. The control panel 106 acquires the inputdisplacement correction value and notifies the control unit 101 thereof(ACT 12).

The control unit 101 calculates, for example, the main scanning printposition of the reference color (K) based on the displacement correctionvalue acquired via the control panel 106 (ACT 13). The control unit 101determines whether or not the calculated main scanning print positionfalls within the range of the valid image area. If the control unit 101determines that the calculated main scanning print position is out ofthe valid image area (ACT 14, YES), the control unit 101 changes theinput displacement correction value to the upper limit value so that thecalculated main scanning print position falls within the range of thevalid image area in the main scanning direction for each color (ACT 15).If the control unit 101 determines that the calculated main scanningprint position is not out of the valid image area (ACT 14, NO), thecontrol unit 101 adopts the input displacement correction value.

Color Shift Correction of Each Color

Next, the color shift correction for each color in the main scanningdirection will be described.

FIG. 4 is a diagram illustrating color shift correction of the imageforming apparatus according to the embodiment and is a diagramillustrating an output example of a test pattern.

In FIG. 4, the print head 116-Y is displaced to the right in the mainscanning direction from a designated center. The print head 116-M isdisplaced to the left in the main scanning direction from the designatedcenter. The print head 116-C is displaced to the right in the mainscanning direction from the designated center. The print head 116-K isdisplaced to the left in the main scanning direction from the designatedcenter. An area commonly covered by these print heads 116-Y, 116-M,116-C, and 116-K is the valid image area. With respect to the validimage area, an uncovered area on the left side of the image in the mainscanning direction is larger than an uncovered area on the right side(e.g., valid image area A is smaller than valid image area B).

The control unit 101 instructs color shift correction at a predeterminedtiming and controls light emission of the print head 116 based on theimage data of the test pattern. For example, the control unit 101periodically instructs color shift correction. Alternatively, thecontrol unit 101 instructs color shift correction in accordance with theelapsed time from previous color shift correction. Since the color shiftof four colors may occur due to a temperature change such asenvironmental temperature, in consideration of this point, the controlunit 101 may instruct color shift correction when the temperature changeamount detected by the temperature sensor 119 exceeds a reference value.The control unit 101 may obtain the temperature change amount bycomparing the temperature measured at the time of performing a previouscolor shift correction with the temperature measured at the time ofperforming printing, and instruct the color shift correction when thetemperature change amount exceeds a reference value.

The image forming unit 115 outputs a test pattern on the transfer belt103 based on an instruction for correcting the color shift and emissioncontrol of the print head 116 according to the image data of the testpattern.

The displacement sensor 117 detects the test pattern output on thetransfer belt 103 and outputs a detected signal to the control unit 101.The control unit 101 detects a valid image area commonly covered by theplurality of print heads based on the detected signal.

Further, the control unit 101 calculates and sets a color shiftcorrection value for correcting the color shift amount, for example, onthe basis of a color having the largest color shift amount. The controlunit 101 calculates the main scanning print position of each color basedon the calculated color shift correction value and determines whether ornot the calculated main scanning print position of each color fallswithin the range of the valid image area. If the control unit 101determines that the calculated main scanning print position of eachcolor is out of the valid image area, the control unit 101 changes thecolor shift correction value to the upper limit value so that thecalculated main scanning print position of each color falls within therange of the valid image area. In addition, the control unit 101 hasdetermined in the displacement correction how many light emittingelements out of the plurality of light emitting elements correspond tothe image in the main scanning direction, but in consideration of thisdetermination result, the control unit 101 calculates and sets a colorshift correction value so that the calculated main scanning printposition of each color falls within the range of the valid image area.

FIG. 5 is a flowchart illustrating an example of color shift correctionof the image forming apparatus according to the embodiment.

As illustrated in FIG. 5, the control unit 101 instructs color shiftcorrection at the predetermined time and controls light emission of theprint head 116 based on the image data of the test pattern. The imageforming unit 115 outputs a test pattern based on the instruction forcorrecting the color shift (ACT 21). That is, the image forming unit 115outputs a test pattern on the transfer belt 103 based on the lightemission control of the print head 116 according to the image data ofthe test pattern.

The displacement sensor 117 detects the test pattern output on thetransfer belt 103. The control unit 101 calculates and sets a colorshift correction value for correcting the color shift amount on thebasis of the color with the largest color shift amount based on thedetection result of the test pattern (ACT 22). The control unit 101calculates the main scanning print position of each color based on thecalculated color shift correction value (ACT 23) and determines whetheror not the calculated main scanning print position of each color fallswithin the range of the valid image area. If the control unit 101determines that the calculated main scanning print position of eachcolor is out of the valid image area (ACT 24, YES), the control unit 101changes the color shift correction value to the upper limit value sothat the calculated main scanning print position of each color fallswithin the range of the valid image area (ACT 25). If the control unit101 determines that the calculated main scanning print position of eachcolor is not out of the valid image area (ACT 24, NO), the control unit101 employs the set color shift correction value.

In the present embodiment, the displacement correction and the colorshift correction have been described, but only one thereof may beexecuted. The control panel 106 may receive inputs of validity orinvalidity of displacement correction and validity or invalidity ofcolor shift correction. When the displacement correction is valid andthe color shift correction is valid, the control unit 101 adjusts thedisplacement correction value and the color shift correction value sothat the main scanning print position of each color falls within thevalid image area.

The control unit 101 may set the validity or invalidity of thedisplacement correction according to the sheet size to be printed. Forexample, the control unit 101 sets the displacement correction to bevalid when an image is formed on a sheet having a first size (a sizeslightly smaller than the length of the print head 116) in the mainscanning direction and controls the light emission of the print heads116-Y, 116-M, 116-C, and 116-K corresponding to each color based on theimage data, displacement correction value, and color shift correctionvalue corresponding to the validity setting. The control unit 101 setsthe displacement correction to be invalid when an image is formed on asheet of a second size (a size sufficiently smaller than the length ofthe print head 116) smaller than the first size in the main scanningdirection, the control unit 101 controls the light emission of the printheads 116-Y, 116-M, 116-C, and 116-K corresponding to each color basedon the image data and the color shift correction value corresponding tothe invalidity setting.

According to the embodiment described above, even if the width of theprint head corresponding to each color is not sufficiently long withrespect to the sheet width (even if the margin that can be secured issmall), it is possible to form an image that is less affected bydisplacement and less affected by color shift. When the displacementcorrection is not required, the displacement correction can be set to beinvalid, and therefore the image forming time can be shortened. When thecolor shift correction is not required, the color shift correction canbe set to be invalid, and therefore the image forming time can beshortened.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of invention. Indeed, the novel apparatus and methods describedherein may be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the apparatus andmethods described herein may be made without departing from the spiritof the inventions. The accompanying claims and their equivalents areintended to cover such forms or modifications as would fall within thescope and spirit of the inventions.

1. An image forming apparatus comprising: an image forming unitconfigured to form an image on a sheet based on image data, the imageforming unit comprising a plurality of print heads arranged in parallel;each of the plurality of print heads configured to emit light through atransfer medium to print a color based on the image data in a validimage area commonly covered by the plurality of print heads; a sensorconfigured to detect a color shift amount of each color to be printed bythe plurality of print heads in a main scanning direction based on atest pattern result printed by the plurality of print heads; and aprocessor configured to: detect the valid image area commonly covered bythe plurality of print heads based on the test pattern result;determine, based on a corresponding color having a largest color shiftamount, a color shift correction value for correcting the color shiftamount, set the color shift correction value for correcting the colorshift amount so that an image of each color in the main scanningdirection falls within a range of the valid image area, and controllight emission of the plurality of print heads based on the color shiftcorrection value and image data.
 2. The image forming apparatusaccording to claim 1, further comprising: an interface configured toacquire a displacement correction value for correcting a displacementamount of the image in the main scanning direction with respect to thesheet, wherein the processor is configured to control the light emissionof the plurality of print heads based on the color shift correctionvalue, the displacement correction value, and the image data.
 3. Theimage forming apparatus according to claim 2, wherein the interface isconfigured to acquire the displacement correction value based on thetest pattern result.
 4. The image forming apparatus according to claim2, wherein the processor is configured to change the displacementcorrection value acquired by the interface so that the image of eachcolor in the main scanning direction falls within the range of the validimage area.
 5. The image forming apparatus according to claim 2, whereinthe interface is configured to set a validity setting as valid orinvalid for displacement correction, and the processor is configured tocontrol the light emission of the plurality of print heads based on thecolor shift correction value, the displacement correction value, and theimage data corresponding to the validity setting.
 6. The image formingapparatus according to claim 5, wherein the processor is configured toset the validity setting of displacement correction to be valid when animage is formed on a sheet of a first size in the main scanningdirection.
 7. The image forming apparatus according to claim 5, whereinthe processor is configured to set the validity setting of displacementcorrection to be invalid when an image is formed on a sheet of a secondsize smaller than the first size in the main scanning direction.
 8. Animage forming method comprising: forming, at an image forming unit, animage on a sheet based on image data by light emission of a plurality ofprint heads through a transfer medium, the print heads being arranged inparallel; detecting, at a sensor, a color shift amount of color to beprinted by each of the plurality of print heads in a main scanningdirection based on a test pattern result printed by the plurality ofprint heads; detect, by a processor, a valid image area commonly coveredby the plurality of print heads based on the test pattern result;setting, by the processor, a color shift correction value for correctingthe color shift amount on the basis of a color having a largest colorshift amount so that an image of each color in the main scanningdirection falls within a range of the valid image area commonly coveredby the plurality of print heads; and controlling, by the processor,light emission of the plurality of print heads based on the color shiftcorrection value and the image data.
 9. The method according to claim 8,further comprising: acquiring, by an interface, a displacementcorrection value for correcting a displacement amount of the image inthe main scanning direction with respect to the sheet; and controlling,by the processor, the light emission of the plurality of print headsbased on the color shift correction value, the displacement correctionvalue, and the image data.
 10. The method according to claim 9, whereinacquiring the displacement correction value comprises determining thedisplacement correction value based on the test pattern result.
 11. Themethod according to claim 9, further comprising adjusting, by theprocessor, the displacement correction value acquired by the interfacesuch that the image of each color in the main scanning direction fallswithin the range of the valid image area.
 12. The method according toclaim 9, further comprising: setting, by the interface, a validitysetting as valid or invalid for displacement correction; andcontrolling, by the processor, the light emission of the plurality ofprint heads based on the color shift correction value, the displacementcorrection value, and the image data corresponding to the validitysetting.
 13. The method according to claim 12, wherein the validitysetting of displacement correction is valid when an image is formed on asheet of a first size in the main scanning direction.
 14. The methodaccording to claim 12, wherein the validity setting of displacementcorrection is invalid when an image is formed on a sheet of a secondsize smaller than the first size in the main scanning direction.